This invention relates to a surface inspection system for detecting defects or imperfections on a surface or surfaces of a hot irradiant material.
Hot direct rolling has recently been widely employed in cogging operations as one of the measures for saving energy. As a result, there is an increasing demand in the art for techniques for detecting objectionable defects on the surface of a hot workpiece, to take the place of the conventional surface flaw detection at room temperature. If it is possible to provide means for detecting defects on the surface of a red hot steel slab which comes out of a cogging mill, the results can be applied for the adjustment of the amount of hot scarfing, hot spot scarfing, classification of quality and feedback to a preceeding process and contribute to the improvement of yield and quality as well as to the economy of manpower and energy. In this connection, there have already been proposed a number of techniques which claim thorough detection of defects on the surface of a hot steel slab on the basis of discernable differences in the amount of irradiant light from the surface being inspected.
Generally, as a hot steel slab of about 1000.degree. C. is evenly cooled in the atmosphere, heat transmission occurs differently depending upon the surface and near surface conditions of the workpiece, showing a different surface temperature at those portions which bear defects or scales as compared with that of the normal or sound portions. More particularly, cracks which show the hotter inside of the material appear brighter while scabs, scales, deposits or other deflects with loose portions which cool off quickly due to low heat transmission appear darker than the remainder.
In most cases, the temperature distribution on the surface of a red hot steel material is not uniform. The surface temperature is lower in the corner portions than in the middle portions of the slab and the uneveness in the surface temperature is also produced by various factors, for example, by the differences in the amount of water which is poured on the slab during the rolling operation. Therefore, it is difficult to distinguish the temperature differences which are attributable to defects from temperature irregularities inherent in the hot steel slab simply by comparing them with a predetermined level.
For automatic detection of defects on the surface of a hot steel slab, the difference in temperature (brightness) of a defective portion from a sound portion has to be detected. Even in the case of eye observation, whether a particular point is defective or not is judged by comparison with the appearance of the circumventive areas. Therefore, it becomes necessary to remove from the detected data the components of the temperature pattern of a normal surface (normal level). The term "normal level" as herein used means the distribution of surface temperature inherent to a hot steel slab free of any surface defects, scales, deposits and the like.
The signals on one scan line are not always at a constant level not only in the inspection at high temperature but also in optical inspection so that it is difficult to detect defect signals by comparison with a predetermined normal level. In order to solve this problem, there have been proposed various methods for producing normal level signals on the basis of input video signals. However, it has been found that the normal level signals produced by the conventional methods are more or less influenced or biased by large dark or bright defects and fail to serve as a rule for measuring the input video signals.
In the scanner type automatic surface inspection of hot steel slabs as mentioned above, the surface of the workpiece which is divided longitudinally and transversely into a multitude of picture elements is scanned by a camera and resulting video signals indicative of the radiation intensity of the respective picture elements are successively processed to extract defect signals which are higher or lower than a normal level. However, as mentioned hereinbefore, the hot steel slabs usually bear, along with real defects, a large number of scales and uneven temperature spots which have radiation intensities confusingly similar to those of the real defects.
In the actual hot rolling operation, it is necessary to inspect for defects not only on the top surface but also on the side and bottom surfaces of the material without allowing any dead zones. However, the current surface inspection systems employ experimentally only one or a few number of overhead cameras due to the difficulties of controlling a number of cameras to maintain the inspecting surfaces within the focusing depths of the respective cameras on all sides of the conveyed material which often makes meandering movements and which considerably differs from one workpiece to another in width and travel position, while protecting the optical and electronic components against the heat radiation of the material and against defilement by scales or deposits which drop from the surfaces of the material.